[support.runtime] # 17 Language support library [[support]](./#support) ## 17.14 Other runtime support [support.runtime] ### [17.14.1](#general) General [[support.runtime.general]](support.runtime.general) [1](#general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6346) Headers[](#header:%3ccsetjmp%3e "17.14.3 Header synopsis [csetjmp.syn]") (nonlocal jumps),[](#header:%3ccsignal%3e "17.14.4 Header synopsis [csignal.syn]") (signal handling),[](#header:%3ccstdarg%3e "17.14.2 Header synopsis [cstdarg.syn]") (variable arguments), and[](cstdlib.syn#header:%3ccstdlib%3e "17.2.2 Header synopsis [cstdlib.syn]") (runtime environment getenv, system), provide further compatibility with C code[.](#general-1.sentence-1) [2](#general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6357) Calls to the functiongetenv ([[cstdlib.syn]](cstdlib.syn "17.2.2 Header synopsis")) shall not introduce a data race ([[res.on.data.races]](res.on.data.races "16.4.6.10 Data race avoidance")) provided that nothing modifies the environment[.](#general-2.sentence-1) [*Note [1](#general-note-1)*: Calls to the POSIX functionssetenv andputenv modify the environment[.](#general-2.sentence-2) — *end note*] [3](#general-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6371) A call to the [setlocale](clocale.syn#lib:setlocale "28.3.5.1 Header synopsis [clocale.syn]") function may introduce a data race with other calls to the setlocale function or with calls to functions that are affected by the current C locale[.](#general-3.sentence-1) The implementation shall behave as if no library function other than locale​::​global calls the setlocale function[.](#general-3.sentence-2) ### [17.14.2](#cstdarg.syn) Header synopsis [[cstdarg.syn]](cstdarg.syn) [🔗](#header:%3ccstdarg%3e) // all freestanding#define __STDC_VERSION_STDARG_H__ 202311Lnamespace std {using [va_list](#lib:va_list "17.14.2 Header synopsis [cstdarg.syn]") = *see below*;}#define va_arg(V, P) *see below*#define va_copy(VDST, VSRC) *see below*#define va_end(V) *see below*#define va_start(V, ...) *see below* [1](#cstdarg.syn-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6396) The contents of the header are the same as the C standard library header [](support.c.headers.general#header:%3cstdarg.h%3e "17.15.1 General [support.c.headers.general]"), with the following changes: - [(1.1)](#cstdarg.syn-1.1) In lieu of the default argument promotions specified in ISO/IEC 9899:2024 6.5.2.2, the definition in [[expr.call]](expr.call "7.6.1.3 Function call") applies[.](#cstdarg.syn-1.1.sentence-1) - [(1.2)](#cstdarg.syn-1.2) The preprocessing tokens comprising the second and subsequent arguments to va_start (if any) are discarded[.](#cstdarg.syn-1.2.sentence-1) [*Note [1](#cstdarg.syn-note-1)*: va_start accepts a second argument for compatibility with prior revisions of C++[.](#cstdarg.syn-1.2.sentence-2) — *end note*] See also: ISO/IEC 9899:2024, 7.16 ### [17.14.3](#csetjmp.syn) Header synopsis [[csetjmp.syn]](csetjmp.syn) [🔗](#lib:jmp_buf) #define __STDC_VERSION_SETJMP_H__ 202311Lnamespace std {using jmp_buf = *see below*; [[noreturn]] void longjmp(jmp_buf env, int val);}#define setjmp(env) *see below* [1](#csetjmp.syn-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6431) The contents of the header are the same as the C standard library header [](support.c.headers.general#header:%3csetjmp.h%3e "17.15.1 General [support.c.headers.general]")[.](#csetjmp.syn-1.sentence-1) [2](#csetjmp.syn-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6435) The function signaturelongjmp(jmp_buf jbuf, int val) has more restricted behavior in this document[.](#csetjmp.syn-2.sentence-1) A setjmp/longjmp call pair has undefined behavior if replacing the setjmp and longjmp by catch and throw would invoke any non-trivial destructors for any objects with automatic storage duration[.](#csetjmp.syn-2.sentence-2) A call to setjmp or longjmp has undefined behavior if invoked in a suspension context of a coroutine ([[expr.await]](expr.await "7.6.2.4 Await"))[.](#csetjmp.syn-2.sentence-3) See also: ISO/IEC 9899:2024, 7.13 ### [17.14.4](#csignal.syn) Header synopsis [[csignal.syn]](csignal.syn) [🔗](#lib:sig_atomic_t) namespace std {using sig_atomic_t = *see below*; // [[support.signal]](#support.signal "17.14.5 Signal handlers"), signal handlersextern "C" using *signal-handler* = void(int); // *exposition only**signal-handler** signal(int sig, *signal-handler** func); int raise(int sig);}#define SIG_DFL *see below*#define SIG_ERR *see below*#define SIG_IGN *see below*#define SIGABRT *see below*#define SIGFPE *see below*#define SIGILL *see below*#define SIGINT *see below*#define SIGSEGV *see below*#define SIGTERM *see below* [1](#csignal.syn-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6485) The contents of the header are the same as the C standard library header [](support.c.headers.general#header:%3csignal.h%3e "17.15.1 General [support.c.headers.general]")[.](#csignal.syn-1.sentence-1) ### [17.14.5](#support.signal) Signal handlers [[support.signal]](support.signal) [1](#support.signal-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6491) A call to the function signal synchronizes with any resulting invocation of the signal handler so installed[.](#support.signal-1.sentence-1) [2](#support.signal-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6495) A [*plain lock-free atomic operation*](#def:plain_lock-free_atomic_operation "17.14.5 Signal handlers [support.signal]") is an invocation of a function f from [[atomics]](atomics "32.5 Atomic operations"), such that: - [(2.1)](#support.signal-2.1) f is the function atomic_is_lock_free(), or - [(2.2)](#support.signal-2.2) f is the member function is_lock_free(), or - [(2.3)](#support.signal-2.3) f is a non-static member function of class atomic_flag, or - [(2.4)](#support.signal-2.4) f is a non-member function, and the first parameter of f has type cv atomic_flag*, or - [(2.5)](#support.signal-2.5) f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or - [(2.6)](#support.signal-2.6) f is a non-member function, and for every pointer-to-atomic argument A passed to f,atomic_is_lock_free(A) yields true[.](#support.signal-2.sentence-1) [3](#support.signal-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6523) An evaluation is [*signal-safe*](#def:evaluation,signal-safe "17.14.5 Signal handlers [support.signal]") unless it includes one of the following: - [(3.1)](#support.signal-3.1) a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe; [*Note [1](#support.signal-note-1)*: This implicitly excludes the use of new and delete expressions that rely on a library-provided memory allocator[.](#support.signal-3.1.sentence-1) — *end note*] - [(3.2)](#support.signal-3.2) an access to an object with thread storage duration; - [(3.3)](#support.signal-3.3) a dynamic_cast expression; - [(3.4)](#support.signal-3.4) throwing of an exception; - [(3.5)](#support.signal-3.5) control entering a [*try-block*](except.pre#nt:try-block "14.1 Preamble [except.pre]") or [*function-try-block*](except.pre#nt:function-try-block "14.1 Preamble [except.pre]"); - [(3.6)](#support.signal-3.6) initialization of a variable with static storage duration requiring dynamic initialization ([[basic.start.dynamic]](basic.start.dynamic "6.10.3.3 Dynamic initialization of non-block variables"), [[stmt.dcl]](stmt.dcl "8.10 Declaration statement"))[191](#footnote-191 "Such initialization can occur because it is the first odr-use ([term.odr.use]) of that variable.") ; or - [(3.7)](#support.signal-3.7) waiting for the completion of the initialization of a variable with static storage duration ([[stmt.dcl]](stmt.dcl "8.10 Declaration statement"))[.](#support.signal-3.sentence-1) A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe[.](#support.signal-3.sentence-2) [4](#support.signal-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L6563) The function signal is signal-safe if it is invoked with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler[.](#support.signal-4.sentence-1) See also: ISO/IEC 9899:2024, 7.14 [191)](#footnote-191)[191)](#footnoteref-191) Such initialization can occur because it is the first odr-use ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule")) of that variable[.](#footnote-191.sentence-1)